Dry eyes and high eye pressure are two common eye conditions that affect millions of people, often leading to concern about a potential connection between them. Dry eyes do not directly cause an increase in eye pressure, as they involve entirely separate structures and physiological functions of the eye. High eye pressure, medically known as intraocular pressure (IOP), is a concern because of its link to a serious sight-threatening condition.
What is Dry Eye Disease
Dry eye disease (DED) is a multifactorial disorder of the tear film and the ocular surface, resulting in uncomfortable symptoms and potential damage to the eye’s outer layers. The tear film is a complex, three-layered structure that coats the cornea and conjunctiva, providing lubrication, protection against pathogens, and a smooth surface for clear vision. This film consists of a mucus layer, a thick middle aqueous layer, and an outermost lipid (oil) layer.
Dry eye occurs when there is a deficiency in the production of the aqueous layer, or more commonly, when the lipid layer is unstable, causing tears to evaporate too quickly. This instability leads to a cycle of inflammation, hyperosmolarity of the tears, and irritation on the eye’s surface. Patients often experience symptoms like a gritty or foreign body sensation, burning, redness, and sometimes excessive watering as a reflex response to the irritation. Dry eye disease is solely an issue of the eye’s external surface and tear quality.
What is Ocular Hypertension and Glaucoma
High eye pressure, or ocular hypertension, is defined as an elevated intraocular pressure (IOP) that is consistently above the normal range, typically considered to be greater than 21 millimeters of mercury (mmHg). The pressure inside the eye is maintained by a constant balance between the production and drainage of a clear fluid called aqueous humor. This fluid is produced by the ciliary body and nourishes the internal structures of the eye before draining out through a mesh-like tissue called the trabecular meshwork.
Ocular hypertension develops when the aqueous humor drainage system becomes compromised or partially blocked, causing the fluid to back up and the internal pressure to rise. Glaucoma is a distinct and more serious condition defined by damage to the optic nerve, which is often a result of sustained, elevated IOP. The increased pressure compresses the delicate nerve fibers, which can lead to irreversible vision loss if left untreated. The process that regulates IOP and leads to glaucoma occurs entirely inside the eye, in the anterior chamber and drainage angle.
Why Dry Eye Does Not Cause High Eye Pressure
The fundamental reason dry eye does not cause high eye pressure lies in the separate physiological compartments involved in each condition. Dry eye disease affects the tear film, which is an external layer on the surface of the cornea and conjunctiva. This external fluid is entirely separate from the aqueous humor, which is the internal fluid regulating IOP.
Think of the eye as a house with two distinct plumbing systems: the external system controls the sprinkler (tears), and the internal system controls the main water pressure (aqueous humor). A problem with the external sprinkler system does not affect the pressure in the internal water pipes. Similarly, a disruption of the tear film on the corneal surface does not influence the production or drainage of aqueous humor within the eye. The mechanisms are completely independent; dry eye is a surface condition, while high eye pressure is an internal fluid dynamic issue.
How Dry Eye Affects High Pressure Measurement
While dry eye does not alter the actual internal eye pressure, it can significantly interfere with the accurate measurement of that pressure. The standard method for measuring IOP is Goldmann applanation tonometry (GAT), which requires the tonometer probe to flatten a small area of the cornea. An intact, stable tear film is necessary for GAT to work correctly, as the measurement relies on the force needed to applanate the eye surface.
In a person with dry eye, the tear film is often unstable, irregular, or rapidly breaking up, which can lead to erroneous IOP readings. A damaged or dry corneal surface can alter the mechanical properties of the cornea, potentially leading to readings that are falsely high or low. For instance, a very dry eye may require a greater force to flatten the corneal surface, which could result in a falsely elevated IOP reading. Furthermore, the lack of a smooth tear film can make the mires, the circles used for alignment during tonometry, indistinct, compromising the precision of the measurement.